1872.] 



Mr. W. TThitehouse on a Xeiv Hygrometer. 



181 



easily renewed when required, nor, indeed, could either of these substances 

 be regarded as wholly free from the interference of frost, as the moisture 

 absorbed from the atmosphere at a temperature much below freezing- 

 point may remain frozen on the surface, and become incapable of con- 

 tinuous measurement. It seemed essential to the accuracy and practical 

 utility of any instrument designed for this purpose : — 



1st. That a fixed and invariable extent of surface should at all times be 

 exposed for absorption of moisture ; 



2nd. That the apparatus should be simple, inexpensive, and not incon- 

 venient in use ; 



3rd. That the hygrometric substance should be continuously and 

 steadily renewable ; and above all, if it were possible, 



4th. That the measurement should be effected thermometrically. 



No solid hygrometric substance seemed capable of meeting these re- 

 quirements ; but all the conditions seemed likely to be fulfilled by the use 

 of concentrated sulphuric acid. This would admit of being spread in an 

 exquisitely fine film over the surface of the bulb of a thermometer by 

 means of a glass capillary siphon, of which one end should rest on the 

 upper part of the bulb, while the other end dipped into a reservoir of the 

 acid. A continuous supply could be maintained for any required length of 

 time by suitable arrangements. The absorption of moisture would neces- 

 sarily be attended by a rise in temperature, and this would be propor- 

 tioned to the amount of hygrometric moisture absorbed ; while the hy- 

 drated acid, having fulfilled its office, would fall in drops from the bulb 

 into any tube or reservoir placed for the purpose. 



An instrument has been constructed by the writer to test this principle, 

 which has, by the courtesy of the Director of the 3Ieteorological Office, 

 been under observation for some weeks. 



It consists essentially of three thermometers of similar construction, and 

 used as a " wet bulb," a " dry bulb," and an " acid bulb," respectively, 

 placed side by side on a suitable frame, and read together for comparison. 



The experience already gained in the use of this instrument has shown 

 that, with a reservoir of proper construction, the supply of acid may be made 

 continuous for any required length of time, and that, from the very slight 

 variations of flow which occur in its action, the supply to the thermometer 

 will be sensibly equable. 



The length of the siphon, and the size of the capillary bore, together 

 with the difference of level between the surface of the fluid in the trough of 

 the reservoir and the point of delivery on the bulb, will determine the rate, 

 of supply of the acid. 



It is clear that either a too rapid and continuous stream of acid at the 

 temperature of the air, or a too scanty supply, would diminish the readings ; 

 yet it is found that practically there may be a pretty wide range of varia- 

 tion in the supply of acid, within which no essential change in the sensibi- 

 lity of the instrument is noticed. 



